1. Field of Applicable Technology
The present invention relates to an apparatus for decoding a multi-level amplitude modulated signal used to convey digital data, such as a QAM signal, and in particular to an improved decoding apparatus whereby accurate decoding is achieved even for the case of the modulated signal being highly distorted, so that such an apparatus becomes applicable to a digital signal magnetic recording and reproduction apparatus.
2. Prior Art Technology
Various proposals have been made in the prior art for implementing a digital signal magnetic recording and reproduction apparatus. Some of such proposals have utilized baseband modulation, with a reduced amount of DC component in the recording signal due to the difficulty of recording and reproducing such a DC component. Such proposals include those utilizing NRZ codes, e.g. by J. K. R. Heitman "An Analytical Approach to the Standardization of Digital Video Tape Recorder", SMPTE J., 91,3, March 1982, also J. K. R. Heitman "Digital Video Recording, New Result in Channel coding and Error Protection", SMPTE J., 93:140-144, February 1984, proposals Using 8-10 block codes, e.g. J. L. E. Baldwin "Digital Television Recording with Low Tape Consumption", SMPTE J., 88:490-492, July 1979, proposals using Miller-square (M.sup.2) codes, e.g. L. Gallo "Signal System Design for a Digital Video Recording System", SMPTE J., 86:749-756, October 1977, proposals using ternary partial response system, etc.
However use of such baseband modulation in a digital signal magnetic recording and reproduction apparatus results in a low level of utilization efficiency of the recording frequency band (i.e. a low value of transmissible bit rate per unit band), due to the basic system of recording the binary signals. For instance, assuming the roll-off rate of a Nyquist transmission system to be 0.5, the utilization efficiency of the frequency band is at most 1.33 bits/sec/Hz. This results in increased tape consumption, making it difficult to achieve sufficiently long recording times.
Various ways of increasing the recording rate have been envisaged in the prior art. These include expanding the recording frequency band, increasing the number of recording channels, or increasing the relative velocity between the tape and recording head. However if the recording frequency band is increased, this will result in a corresponding decrease in the S/N ratio, so that a substantial increase in recording rate cannot be achieved. If the number of recording channels is increased, then the track width of each channel must of course be narrowed, so that this again leads to a decrease in the S/N ratio. If the tape/head relative velocity is increased, then this will result in problems with regard to increased tape consumption.
Error control coding can be applied to counteract such deterioration of the S/N ratio, by providing an improvement in the error rate. However such error control coding results in a reduction of the bit rate of the actual data that can be recorded. Such error control coding is described for example by L.M.H.E. Dreissen et al., "An Experimental Digital Video Recording System" IEEE Conf. June 1986, or by C. Yamamitsu et al., "An Experimental Digital VTR Capable of 12-hour Recording", IEEE Trans. on CE, CE-33, No. 3, pp 240-248, 1987.
With all of the above prior art proposals, video data are recorded on tape in the form of a digital recording signal. It has also been been proposed in the prior art to combine a multi-level modulation scheme (which provides a high utilization efficiency of the transmission frequency band) with an error control code, for digital data transmission (e.g. G. Ungerboeck "Channel Coding with Multilevel/Phase Signals", IEEE Trans. on IT, IT-28, No. 1, pp. 55-67, 1982). However such a system would not be directly applicable to a magnetic recording and reproduction apparatus, due to special conditions which arise in magnetic recording, i.e. the effects of non-linear distortion and saturation characteristics of the magnetic recording medium upon the reproduced (playback) signal.
The assignees of the present invention have previously proposed, in U.S. patent application Ser. No. 07/251,094 (1988,9,29), a multi-level modulation method providing a high recording rate and high utilization efficiency of the recording frequency band, for a digital signal magnetic recording and reproduction apparatus. However that proposal does not sufficiently take into consideration the effects of amplitude non-linearity which arises in the recording/reproduction process.
The problems which arise when applying to a magnetic recording and reproduction apparatus multi-level modulated signal as commonly used for digital data transmission will be summarized in the following using as an example 16 QAM (i.e. 16-state quadrature amplitude modulation), which is a widely utilized form of multi-level modulation. The term "multi-level modulation signal" as used herein refers to the class of modulation signals in which respective data values are expressed in each of successive symbol periods as a combination of quantized carrier amplitude and phase values, such as QAM or PSK signals. With 16 QAM, a modulation signal is generated which can be considered to consist of two carrier signal components that are in phase quadrature (the I and Q components), these being mutually independently amplitude-modulated at four fixed levels. Thus, one of a total of 16 possible symbols, or code values, can be expressed in each symbol period of a 16 QAM signal. When such a 16 QAM signal is received, the I and Q carrier components are separately demodulated, and the resultant demodulated I and Q signals are then sampled at appropriate time points, to determine the respective amplitude levels at the sampling times, i.e. a pair of demodulated I and Q amplitude values is obtained at each sampling time. In the prior art, discrimination of the respective codes represented by these demodulated I, Q pairs has been based on the fixed correspondence between the code values (usually, 4-bit binary code values) and the I, Q pairs that were used at the time of modulation. This can be understood by referring to FIG. 1, which is a diagram of the I-Q plane showing an example of a symbol constellation (i.e. array of signal reference points represented in the I-Q plane) of a 16 QAM signal. At the time of modulation, each code value of the signal to be recorded (e.g. the binary codes 0000 to 1111) is assigned a specific combination of states of the I, Q signals, these combinations being represented as a set of 16 reference points which in this example are spaced equidistantly in the I-Q plane, with one of these points being designated by numeral 201. At the time of modulation, the point 201 (i.e. a specific pair of I, Q values) might for example represent a binary code value 1011 of a digital signal that is to be recorded on a magnetic recording medium. When the QAM signal is subsequently reproduced from the recording medium, then as a result of amplitude distortion arising in the recording and reproduction process, the pair of I, Q values (originally corresponding to the point 201) that are obtained by demodulation of the QAM signal will now correspond to a different point in the I-Q plane, e.g. point 203. In that case no problem will arise with a prior art method of QAM signal decoding, in which all points (I, Q pairs) that are closer to the point 201 than to any other of the symbol constellation points (i.e. that fall within the hatched-line rectangle 202) will be discriminated as representing the code value that has been assigned to the point 201. Thus, no error will arise in such a case. However with a magnetic recording and reproduction apparatus, a high degree of amplitude distortion (in addition to phase jitter) arises in the recording and reproduction process. Thus in some cases an I, Q data value pair in the reproduced signal, which should correspond to the constellation point 201, may be displaced to a position in the I-Q plane which is outside the rectangle 202, i.e. it becomes impossible for the system to discriminate that I, Q pair as representing the same binary code value as that assigned to point 201. An incorrect binary code value will thus be outputted in response to that I, Q pair by a prior art decoding apparatus.
Thus, with a prior art apparatus for decoding a 16 QAM signal, whose operation is based upon a set of fixed reference data (specifically, the 16 reference points 201 etc. in FIG. 1), the high level of amplitude distortion that is introduced in the recording and reproduction process of a magnetic recording and reproduction apparatus will result in a high error rate.
Although the above problem has been described for the particular case of 16 QAM using a rectangular symbol constellation, such a problem will also occur in the case of other types of multi-level modulation method used for digital data transmission, such as 8 PSK (8-state phase shift keying) modulation, having other forms of symbol constellation.